I may have misled you. My apologies. BringToFront applies to controls but not, apparently to forms. That is why you get the comment about having to be in design mode.
Open the form that you want "in front" in acDialog mode, which makes it a "modal" form. IF you do this, you stop all background activities within Access because the world stops for dialogs. Note, however, that if you open another form from the dialog form, if that new form is not ALSO in dialog mode, it will be behind the dialog form.
If you have multiple non-dialog forms, what SHOULD happen is that if you set focus on any control on the desired form, that SHOULD "bubble up" to the top. If any of the forms are in dialog mode, though, dialog mode overrides the effect of set focus.
Perhaps you could use Forms("formname").ControlName.SetFocus to force the named form to the front after you open it. Or if you want a given form to always go in front when you first open it, you can Me.ControlName.SetFocus from the form's _Load event. If you use the .SetFocus method to bring forms forward, remember that the form in front is the one with the chronologically most recent .SetFocus method.